The structure and dissolution behaviour of Na 2 O•CaO•(15-x)Al 2 O 3 •xB 2 O 3 •SiO 2 and Na 2 O•MgO•(15x)Al 2 O 3 •xB 2 O 3 •SiO 2 glasses, relevant to compositions of UK nuclear waste glass, have been investigated using nuclear magnetic resonance (NMR) spectroscopy and static dissolution experiments using the PCT protocol. Structural data from 11 B, 27 Al and 29 Si NMR analyses show that increasing the Increasing the [B 2 O 3 ]/([Al 2 O 3 ]+[B 2 O 3 ]) ratio of the alkali-alkaline-earth aluminoborosilicate glasses led to an overall decrease in the proportion of non-silicate tetrahedral species (IV Al + IV B) and a decrease in Si-OX bonds (X = B, Al). The Mg-containing glasses exhibited lower IV B fractions than their Ca-containing counterparts, which is thought to be due to the presence of IV Mg tetrahedra in the network. The measured corrosion rates were similar for both Ca and Mgcontaining glasses although unexpectedly some Ca-containing glasses exhibited higher corrosion losses than the Mg-containing ones for time periods up to 112 d. However, there was evidence of a greater tendency to rate resumption in the Mg containing than the Ca containing ones. Alteration products were found to contain Ca, Si and Al with the Ca containing glasses and Ca, Mg, Si and Al with the Mg containing glasses; Na was not detected in the alteration products although its presence cannot be ruled out based on the data obtained.

The composition and structure of the mixed metal molybdates that can form in simulated high level nuclear waste (HLW) glass melts have been studied. It was found that molybdates of a tetragonal scheelite type were formed upon heat treatment of the simulated glass samples (representative of the slow cools experienced by the real vitrified product), and that these compounds are capable of incorporating the majority of the mono, di, and trivalent cations that would be present in a real HLW glass. In addition, it has been shown that altering the redox conditions prevailing upon melting can promote or suppress crystallisation in simplified model waste glasses that contain molybdenum. Experiments to investigate the effect of redox conditions during melting of simulated HLW glass on molybdate formation are also reported.

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The highly heterogeneous nature of UK legacy damaged and degraded spent nuclear fuels and so called, ‘orphan fuels’, prohibits the use of standard conditioning methods. An inventory of UK residual fuels yielded an account for three main fuel types: Magnox, AGR (advanced gas-cooled reactor) and MOx (mixed oxides). A series of glass and ceramic type host systems have been investigated for potential conditioning of these high uranium content spent fuel materials. Electron microscopy and powder X-ray diffraction techniques were used to characterise the prototypical wasteforms. Two sets of low-melt temperature phosphate glass compositions were trialled with additions of CeO2 to simulate the fluorite structure and large ionic radius of U in oxide fuels. Evolution of monazite-type phases at simulant oxide fuel loadings above 15 wt.% highlighted a potential development into a glass-ceramic hybrid assemblage. Investigation into the use of an alkoxide nitrate synthesis route for SYNROC-F type ceramic precursors has allowed for the demonstration of a sintered host pyrochlore phase containing up to ∼40 wt.% fuel simulant CeO2. Gas evolution has led to increased porosity at higher temperatures and longer sintering times, this may be mitigated by higher pre-calcination temperatures.